Measurement and monitoring of blood flow velocity is commonly carried out using ultrasound Doppler technology. Such flow measurements are primarily directed to peripheral arteries such as the carotid and femoral arteries as well as intra skull arteries by Trans-Cranial Doppler (TCD). Some devices capable in principle of measuring blood flow in the coronary arteries are available on the market. Many such cardiac devices are based on ultrasound cardiac imaging with the addition of Doppler velocity flow measurement capability which can be used for the coronary artery flow measurements. However the use of these devices is limited, due in part to the complexity and low sensitivity of the available technologies and measuring routines.
Two conventional modes are popular. The first mode is color Doppler imaging which, when directed appropriately, can display a very low resolution image of the flow direction with velocity indicated on a low resolution color scale. This mode is primarily designed to monitor flow directions rather than an accurate flow velocity monitoring vs. time.
In the second mode, the cardiac echo system is operated in imaging mode and the desired coronary artery is manually searched for. Echo operators that are highly trained for this task can identify a coronary artery, usually in cross section, and then direct to it a pointer that selects the identified area for using Doppler mode. In this case the coronary blood flow velocity at the selected point is displayed in real time. In these systems, a real time graphical display is generated in which the Y axis represents velocity and the X axis represents the running time. A low resolution indication of the power of the reflected ultrasound energy is provided using a gray scale (by assigning different intensities to different powers) or a color scale (by assigning different colors to different powers). The velocity of all elements, which move relative to the ultrasound beam, (for example, erythrocytes that flow at different velocities at different locations in the coronary artery, cardiac wall movements, cardiac valve movements, blood flow out off or into the heart chambers, etc.) at a give time are plotted along a bar or line (parallel to the Y axis) such that their position represents to their respective velocities. Ultrasound reflections from different moving objects having similar velocities (Doppler shifts), at any given time, are represented by the same display point.
FIG. 1 is an example of such a Doppler flow velocity display, which is a conventional display of blood flow velocity (Y axis) recorded from the left anterior descending coronary artery as a function of time (X axis). The power of the received ultrasound signal is given by the color of the points according to the color scale (in dB) depicted at the right of the display. Note that since this application is being submitted in black-and white, the color information is reproduced as grayscale.
A disadvantage of this type of display is that it often does not contain enough information to distinguish between signals originating from different sources that have similar velocities. Thus, the flow velocity pattern of the specific desired target region may not be identified and separated from other signals. The ambiguity in the display interpretation is illustrated in the example given in FIGS. 2A and 2B. The tracings displayed in those two figures of flow velocity signals for two different patients, with R-wave information added to the display (e.g., by superimposing an ECG display on the flow display). The signals denoted with numbers 1-5 may be individual entities, however, this can not be verified. Note that in FIGS. 2A and 2B, “R” denotes the position in time of the R wave of the ECG.
In FIGS. 2A and 2B, Signals 1, 2 & 3 (in FIG. 2A), and 4 & 5 (in FIG. 2B) may represent either coronary flow in the LAD (Left Anterior Descending coronary) during the diastole, flow through the mitral valve into the left ventricle, or heart valve movements, all of which are moving in the same direction (relative to the probe) and are located in the ultrasound beam. Separation between such Doppler signals could theoretically be achieved by pulsed Doppler with Gating that provides information regarding the depth of the reflecting object, (i.e., the distance from the ultrasound probe). Unfortunately, such separation is often impossible in practice as the objects may appear in the same gates. Currently this issue can sometimes be resolved by the use of imaging color Doppler. Here the user can try to include in a selected frame the desired target alone. Once this is achieved, he or she switches to Doppler mode and takes the measurement. This procedure, however, is cumbersome and often unsuccessful. Moreover, as the method relies on an initial imaging stage, the measurements are restricted to a relatively small number of windows where the “view” is not masked by bone (e.g., the ribs & sternum) or lung tissue.